Task: Fix AI design video duration selector

Identity

Audit and fix the AI Design video confirmation duration selector in src/pages/ImageCanvas/index.tsx.
Defensively expose only enabled server-provided duration options from 2 through 6 seconds, including when an older server Quote still selects or advertises 7-15 seconds.
Verify that selecting 6 seconds survives Quote repricing and is sent by both the Quote PATCH and final confirmation request.
Add focused Renderer regression coverage in tests/unit/image-canvas-page.test.tsx and verify the existing Main-to-Works snake-case request mapping in tests/unit/works-square-design-workspace.test.ts.

Own only src/pages/ImageCanvas/index.tsx, tests/unit/image-canvas-page.test.tsx, tests/unit/works-square-design-workspace.test.ts, and this task record.
Preserve Conversation-owned Quote state, server-authoritative repricing, and existing request field naming across Renderer and Main.
Do not modify the Works Square server, Electron Main routes/adapters, unrelated AI Design interactions, or files owned by peer tasks.
Keep invalid legacy selected durations visibly unselected until the user chooses an allowed value; do not silently submit a replacement duration without a fresh server Quote.

The confirmation card now filters enabled server-provided video durations to the inclusive 2-6 second client boundary.
A legacy Quote whose selected duration is outside that boundary renders an explicit disabled 请选择时长 placeholder instead of visually selecting one allowed value while retaining a different hidden draft value.
Selecting 6 seconds updates the controlled draft, survives the debounced server re-quote, and is used by final confirmation.
Final-review P1 follow-up: a video Quote with null, an out-of-range duration, a disabled duration, or a duration absent from the filtered server options now disables the confirmation button and is rejected again inside the shared handleConfirm path used by direct, quick-reply, and text confirmation.
Only an explicit allowed selection followed by a successful server re-quote re-enables confirmation.
Existing request seams were verified: Renderer sends durationSeconds: 6, and the Main cloud adapter sends duration_seconds: 6 to Works Square.

Red test: the new legacy-duration regression initially failed because the native select visually resolved the unmatched controlled value 10 to option 2.
corepack pnpm exec vitest run tests/unit/image-canvas-page.test.tsx -t "limits legacy video durations" — passed after the fix (1 passed, 46 skipped).
corepack pnpm exec vitest run tests/unit/image-canvas-page.test.tsx tests/unit/image-workspace-api.test.ts tests/unit/works-square-design-workspace.test.ts — passed (3 files, 92 tests).
corepack pnpm run typecheck — passed.
corepack pnpm exec eslint src/pages/ImageCanvas/index.tsx tests/unit/image-canvas-page.test.tsx tests/unit/works-square-design-workspace.test.ts — passed with no output.
corepack pnpm run build:vite — passed for Renderer, Electron Main, and Preload; only the existing dynamic-import and chunk-size warnings were reported.
git diff --check — passed; Git reported only the repository's LF-to-CRLF checkout warning.
Final-review P1 red test: the legacy 10-second Quote initially left 确认并开始生成 enabled.
Final-review P1 focused regression: legacy 10-second initial state disables confirmation and text confirmation sends no request; selecting 6 seconds, completing re-quote, and confirming sends 6 — passed.
Post-P1 focused suite: 3 files / 92 tests passed; typecheck and scoped ESLint passed; Renderer/Main/Preload build:vite passed with the same existing warnings.

Production acceptance still requires a real server Quote containing the supported duration options; the client intentionally does not synthesize missing 2-6 second choices.